Test of the lower mantle slab penetration hypothesis using broadband S waves

Broadband S and ScS displacement pulses from deep‐focus earthquakes in the Sea of Okhotsk, recorded at analog and digital stations in North America and Europe, show complexities produced by velocity heterogeneity in the lower mantle. The slab beneath the Sea of Okhotsk strikes N40°E, while the stations used in this study range from N40°W to N70°E, allowing a test of the hypothesis that multipathing in a lower mantle slab extension gives rise to the waveform complexities. Comparisons of the S and ScS displacement pulses at a given station indicate that in many cases the direct S pulses have a broader, more complex waveform than the ScS pulses. We parameterized the waveform complexity by the energy temporal centroid difference (C_S‐C_ScS) between the S and ScS pulses. The largest C_S‐C_ScS anomalies are observed along the strike of the slab. There is also a correlation between the C_S‐C_ScS values and ScS‐S travel time residuals for stations along the strike of the slab and no correlation for off‐strike azimuths. However, C_S‐C_ScS anomalies are observed over the entire azimuth range, including azimuths to the southwestern U.S., for which the raypaths should quickly exit any deep slab structure. These data suggest that lateral velocity heterogeneities both near the source (possibly due to slab penetration) and deep in the mantle near the S wave turning points contribute to the S and ScS complexity.